The use of compressed air, for example in industrial engineering or medical engineering, is always connected with a quality problem as regards humidity in the pipe network and at the points of consumption. An important requirement therefore consists in drying the compressed air. To this end diaphragm filters are used, which are selectively permeable to water vapour. The filter housing has a bundle of highly-selective hollow fibre diaphragms arranged in it, through which humid compressed air flows. The humid compressed air is preferably filtered in order to trap remaining dirt particles, oil mist and condensate which could block the hollow fibre diaphragms. The hollow fibre diaphragms allow the water vapour to diffuse to the outside. At the outlet for dried compressed air a small partial current of the compressed air is branched off and used as sweep gas after expansion. The sweep gas is passed across the outside of the hollow fibres in a counter-current to the compressed air. Due to the difference in water vapour concentration a continuous migration of water molecules from the compressed air into the sweep gas is achieved. Thus the commonly used term “diaphragm filter” can be at least misunderstood insofar as a diaphragm filter does not perform separation mechanically but by diffusion. For simplicity's sake this expression is nevertheless used in terms of the disclosure.
This process is continuous. The sweep gas constantly dries the entering humid compressed air. Only water molecules can penetrate through the diaphragms of the hollow fibres. The composition of the dried compressed air remains unchanged. As a result pure, dry compressed air is obtained.
The volume of the sweep gas current is defined by a nozzle which allows a constant volume current defined by pressure and size of nozzle to pass through. The disadvantage is that in case of a partial load or zero load, when there is little or no demand for compressed air, the sweep gas current remains constant although it is not at all needed or needed only to a limited extent. This leads to considerable pressure losses in the overall system.
In order to alleviate this problem, conventional sweep gas regulators adapt the sweep gas current to the required quantity.
U.S. Pat. No. 5,160,514 has disclosed a drying apparatus for compressed air with such a sweep gas regulator, which is integrated with a drying apparatus for compressed air. To this end the compressed air outlet of the drying apparatus is in communication with a chamber, which is limited by a diaphragm. On the other side of the diaphragm there is a further chamber, which is connected via a line to the compressed air inlet of the drying apparatus. The diaphragm assumes a position which depends on the pressure difference between compressed air inlet and compressed air outlet. The diaphragm is connected to a valve element which comprises a lateral slot and is guided in an associated inner bore of the housing. Depending on the position of the diaphragm the valve element assumes a position, which allows a current of compressed air to flow more or less strongly from the compressed air outlet of the drying apparatus in direction of the sweeping line.
This sweep gas regulator does not function in as satisfactory manner. In particular when there are small pressure differences between inlet pressure und outlet pressure for the compressed air of the drying apparatus, regulating behaviour is not satisfactory. Further when there are large pressure differences between inlet pressure und outlet pressure of the compressed air of the drying apparatus there exists the problem that the air at the compressed air outlet must on the one hand supply pressure and on the other hand must flow. A disadvantage with this solution is that regulation must be adjusted to supply a concrete (average) operating pressure. If operating pressure fluctuates regulating behaviour changes distinctly. This leads to conditions, which are confusing.
Another factor is that the differential pressure is also dependent on the filter element load, the degree of filling impacts on the differential pressure and thus on the regulation of the sweep gas. This can change during the useful life of the filter element, which is undesirable.